Tunnel barriers based on alkaline earth oxides

ABSTRACT

Magnetic tunnel junctions are disclosed that include ferromagnetic (or ferrimagnetic) materials and a bilayer tunnel barrier structure that includes a layer of alkaline earth oxide. The bilayer also includes a layer of crystalline material, such as MgO or Mg—ZnO. If MgO is used, then it is preferably ( 100 ) oriented. The magnetic tunnel junctions so formed enjoy high tunneling magnetoresistance, e.g., much greater than 100% at room temperature.

TECHNICAL FIELD

The invention relates to an improved tunnel barrier for use inspintronic devices such as injectors of spin polarized current and themagnetic tunnel junction (MTJ). MTJ magnetoresistive (MR) devices finduse as magnetic field sensors such as in read heads for readingmagnetically recorded data, as memory cells in nonvolatile magneticrandom access memory (MRAM) cells, and for magnetic logic and spintronicapplications. More particularly, this invention relates to a method offorming improved composite tunnel barriers formed from alkaline-earthoxides and oxides of Mg, Al and Zn.

BACKGROUND OF THE INVENTION

The basic component of a tunnel spin injector and a magnetic tunneljunction is a ferromagnetic layer combined with a tunnel barrier. Thebasic structure of an MTJ is a sandwich of two thin ferromagnetic and/orferrimagnetic layers separated by a very thin insulating layer. In boththe spin injector and the MTJ, the electrons that tunnel from theferromagnetic electrode across the tunnel barrier are spin polarized.The degree of spin polarization depends on both the composition andnature of the ferromagnetic metal, the tunnel barrier, and the interfacebetween the two. In an MTJ the tunneling current is typically higherwhen the magnetic moments of the ferromagnetic (F) layers are paralleland lower when the magnetic moments of the two ferromagnetic layers areanti-parallel. The change in conductance for these two magnetic statescan be described as a magneto-resistance. Here the tunnelingmagnetoresistance (TMR) of the MTJ is defined as (R_(AP)−R_(P))/R_(P)where R_(P) and R_(AP) are the resistance of the MTJ for parallel andanti-parallel alignment of the ferromagnetic layers, respectively. MTJdevices have been proposed as memory cells for nonvolatile solid statememory and as external magnetic field sensors, such as TMR read sensorsfor heads for magnetic recording systems. For a memory cell application,one of the ferromagnetic layers in the MTJ is the reference layer andhas its magnetic moment fixed or pinned, so that its magnetic moment isunaffected by the presence of the magnetic fields applied to the deviceduring its operation. The other ferromagnetic layer in the sandwich isthe storage layer, whose moment responds to magnetic fields appliedduring operation of the device.

In the quiescent state, in the absence of any applied magnetic fieldwithin the memory cell, the storage layer magnetic moment is designed tobe either parallel (P) or anti-parallel (AP) to the magnetic moment ofthe reference ferromagnetic layer. For a TMR field sensor for read headapplications, the reference ferromagnetic layer has its magnetic momentfixed or pinned so as to be generally perpendicular to the magneticmoment of the free or sensing ferromagnetic layer in the absence of anexternal magnetic field. The use of an MTJ device as a memory cell in anMRAM array is described in U.S. Pat. No. 5,640,343. The use of an MTJdevice as a MR read head has been described in U.S. Pat. Nos. 5,390,061;5,650,958; 5,729,410 and 5,764,567.

FIG. 1A illustrates a cross-section of a conventional prior-art MTJdevice. The MTJ 100 includes a bottom “fixed” or “reference”ferromagnetic (F) layer 15, an insulating tunnel barrier layer 24, and atop “free” or “storage” ferromagnetic layer 34. The MTJ 100 has bottomand top electrical leads 12 and 36, respectively, with the bottom leadbeing formed on a suitable substrate 11, such as a silicon oxide layer.The ferromagnetic layer 15 is called the fixed (or reference) layerbecause its magnetic moment is prevented from rotating in the presenceof an applied magnetic field in the desired range of interest for theMTJ device, e.g., the magnetic field caused by the write current appliedto the memory cell from the read/write circuitry of the MRAM. Themagnetic moment of the ferromagnetic layer 15, whose direction isindicated by the arrow 90 in FIG. 1A, can be fixed by forming it from ahigh coercivity magnetic material or by exchange coupling it to anantiferromagnetic layer 16. The magnetic moment of the freeferromagnetic layer 34 is not fixed, and is thus free to rotate in thepresence of an applied magnetic field in the range of interest.

In the absence of an applied magnetic field, the moments of theferromagnetic layers 15 and 34 are aligned generally parallel (oranti-parallel) in an MTJ memory cell (as indicated by the double-headedarrow 80 in FIG. 1A) and generally perpendicular in a MTJmagnetoresistive read head. The relative orientation of the magneticmoments of the ferromagnetic layers 15, 34 affects the tunneling currentand thus the electrical resistance of the MTJ device. The bottom lead12, the antiferromagnetic layer 16, and the fixed ferromagnetic layer 15together may be regarded as constituting the lower electrode 10. In FIG.1B, the reference ferromagnetic layer 15 is replaced by a sandwich oftwo ferromagnetic layers 18 and 19 antiferromagnetically coupled througha metallic spacer layer 17 as shown by the MTJ 100′ of FIG. 1B. Thelower electrode is now given by the reference numeral 10′, and themagnetic orientation of the layers 18 and 19 is given by the arrows 90′and 95, respectively.

MTJs can display large tunneling magnetoresistance (TMR) at roomtemperature of up to 70% using Al₂O₃ tunnel barriers and more than 220%using MgO tunnel barriers (S. S. P. Parkin et al., Nature Materials 3,862 (2004)). The resistance of the MTJ depends on the relativeorientation of the magnetizations of the F electrodes. Here we defineTMR=(R_(AP)−R_(P))/R_(L) where R_(AP) and R_(P) correspond to theresistance for anti-parallel and parallel alignment of the F electrodes'magnetizations, respectively, and R_(L) is the lower of either R_(P) orR_(AP). The TMR originates from the spin polarization of the tunnelingcurrent which can be measured most directly using superconductingtunneling spectroscopy (STS) in related tunnel junctions in which one ofthe ferromagnetic electrodes of the MTJ is replaced by a thinsuperconducting (S) layer. The TMR and spin polarization are then simplyrelated according to Julliere's model (M. Juliere, Phys. Lett. 54A, 225(1975)).

For applications of magnetic tunnel junctions for either magneticrecording heads or for non-volatile magnetic memory storage cells, highTMR values are needed for improving the performance of these devices.The speed of operation of the recording head or memory is related to thesignal to noise ratio (SNR) provided by the MTJ—higher TMR values willlead to higher SNR values for otherwise the same resistance. Moreover,for memory applications, the larger the TMR, the greater is thedevice-to-device variation in resistance of the MTJs that can betolerated. Since the resistance of an MTJ depends exponentially on thethickness of the tunneling barrier, small variations in thickness cangive rise to large changes in the resistance of the MTJ. Thus high TMRvalues can be used to mitigate inevitable variations in tunnel barrierthickness from device to device. The resistance of an MTJ deviceincreases inversely with the area of the device. As the density ofmemory devices increases in the future, the thickness of the tunnelbarrier will have to be reduced (for otherwise the same tunnel barriermaterial) to maintain an optimal resistance of the MTJ memory cell formatching to electronic circuits. Thus a given variation in thickness ofthe tunnel barrier (introduced by whatever process is used to fabricatethe MTJ) will become an increasingly larger proportion of the reducedtunnel barrier thickness and so will likely give rise to largervariations in the resistance of the MTJ device.

Different tunnel barrier materials have distinct advantages anddisadvantages. For example, MgO tunnel barriers exhibit high tunnelingmagnetoresistance and tunneling spin polarization, have very highthermal stability and have relatively low resistance for the same tunnelbarrier thickness as compared to, for example, aluminum oxide tunnelbarriers. A potential disadvantage of crystalline MgO tunnel barriers isthat the magnetic properties of the free or sensing magnetic layer,adjacent to the MgO barrier, may be influenced by the crystallinity ofthe MgO layer, leading possibly to greater variations in magneticswitching fields, from device to device, than are seen using amorphousbarriers with no well defined crystallographic structure. However, thiscan be mitigated by the use of amorphous ferromagnetic electrodes.Another potential disadvantage of both MgO and alumina tunnel barriersis that they have high tunnel barrier heights: The tunnel barrier heightis related to the electronic band gap of the insulating material, andthe band gaps of MgO and alumina are high. For applications where thedevice size is deep sub-micron in size and for ultra high speedapplications, such as for advanced magnetic recording read headelements, lower tunnel barrier heights may be advantageous since theseallow for lower resistance-area products or for thicker tunnel barrierswith the same resistance-area product.

What is needed are tunnel barrier materials which give rise tosubstantial tunneling magnetoresistance at low resistance-area productsand, which, when formed on magnetic electrodes, do not substantiallyoxidize the underlying magnetic electrode, which would otherwise depressthe magnitude of the spin polarization of the tunneling current and thetunneling magnetoresistance of magnetic tunnel junctions using thesetunnel barriers.

SUMMARY OF THE INVENTION

One embodiment of the invention is a device that includes a tunnelbarrier structure. The structure includes a first layer of an alkalineearth oxide tunnel barrier and a second layer of at least one of acrystalline MgO tunnel barrier and a crystalline Mg—ZnO tunnel barrier.The tunnel barrier structure is in contact with an underlayer. Theunderlayer, the first layer, and the second layer are in proximity witheach other, thereby enabling spin-polarized charge carrier transportbetween the underlayer and the first and second layers. The alkalineearth oxide layer (which may be crystalline) may advantageously includeat least one of Ca, Sr, and Ba. The second layer may advantageouslyinclude crystalline grains that are substantially (100) oriented. Also,the tunnel barrier structure may further include an Al₂O₃ tunnelbarrier, which is advantageously amorphous. The underlayer may include asemiconductor, e.g., at least one of Si and GaAs. Alternatively, theunderlayer may include material selected from the group consisting offerrimagnetic materials and ferromagnetic materials, and the device mayfurther include an overlayer that likewise includes a material selectedfrom the group consisting of ferrimagnetic materials and ferromagneticmaterials; in this case, the underlayer, the tunnel barrier structure,and the overlayer form a magnetic tunnel junction.

One aspect of the invention is a method of forming the device. In thismethod, the second layer is formed by 1) depositing at least one firstmetal onto a surface of the underlayer to form a metal layer thereon, inwhich the surface is substantially free of oxide and 2) directing atleast one second metal, in the presence of oxygen, towards the metallayer to form a metal oxide tunnel barrier in contact with theunderlayer, in which the oxygen reacts with the second metal and themetal layer, and in which the metal oxide tunnel barrier includes Mg.The first layer is formed by forming an alkaline earth oxide tunnelbarrier over the metal oxide tunnel barrier. The method may furtherinclude annealing the metal oxide tunnel barrier to improve itsperformance. The metal oxide tunnel barrier may include at least one ofa MgO tunnel barrier and a Mg—ZnO tunnel barrier.

Another aspect of the invention is yet another method of forming thedevice. In this method, at least one first metal that includes Mg isdeposited onto a surface of the underlayer to form a metal layerthereon, in which the surface is substantially free of oxide. The methodalso includes directing at least one second metal that includes analkaline earth element, in the presence of oxygen, towards the metallayer to form a bilayer in contact with the underlayer. The bilayer soformed includes the first layer and the second layer.

Yet another embodiment of the invention is a device that includes afirst magnetic layer and a second magnetic layer, in which each of thefirst and second magnetic layers includes a material selected from thegroup consisting of ferrimagnetic materials and ferromagnetic materials.The device also includes a first tunnel barrier layer of an alkalineearth oxide tunnel barrier and a second tunnel barrier layer of at leastone of a crystalline MgO tunnel barrier and a crystalline Mg—ZnO tunnelbarrier. The first and second tunnel barrier layers form a bilayer oftunnel barriers. The first magnetic layer, the tunnel barrier bilayer,and the second magnetic layer form a magnetic tunnel junction. Thedevice advantageously has a tunneling magnetoresistance at roomtemperature of greater than 50%, 100%, 200%, or even 300%. The first andsecond magnetic layers may each include a Co—Fe alloy, and the alkalineearth oxide preferably includes an oxide of at least one of Ca, Sr, andBa.

The MgO and Mg—ZnO tunnel barriers of the magnetic tunnel junctiondevices disclosed herein are preferably prepared according to methods inwhich the lower ferromagnetic (or ferrimagnetic) electrode is notoxidized, so as to give much higher tunnel magnetoresistance values thanin the prior art using other tunnel barrier material such as aluminumoxide. Similarly, much higher spin polarization values of tunnelingcurrent are obtained in tunnel junction devices with one or moreferromagnetic (or ferrimagnetic) electrodes. The MgO or Mg—ZnO tunnelbarrier so formed does not have a significant number of defects thatwould otherwise lead to hopping conductivity through the tunnel barrier.In preferred methods, highly oriented (100) MgO or Mg—ZnO barriers areformed without using single crystalline substrates or high depositiontemperatures, thereby facilitating the manufacture of devices usingstandard deposition techniques on polycrystalline or amorphous films.Post anneal treatments are preferred to improve the tunnelingmagnetoresistance, which for the MgO structures disclosed herein canexceed 50, 100, 150 or even 200% at room temperature, and which for theMg—ZnO structures disclosed herein can exceed 50% at room temperature.

For several aspects and embodiments of the invention disclosed herein, aMgO or Mg—ZnO tunnel barrier is sandwiched between an underlayer and anoverlayer, either one or both of which may include one or more layers ofa ferromagnetic material and/or a ferrimagnetic material. While the MgO(or Mg—ZnO) tunnel barrier is preferably in direct contact with theferromagnetic material and/or ferrimagnetic material, each of theunderlayer and overlayer may optionally include one or more spacerlayers which are adjacent to the tunnel barrier but which do notsignificantly affect the tunneling properties of the MgO (or Mg—ZnO)layer, e.g., by not significantly diminishing the spin polarization ofelectrons tunneling through the tunnel barrier. For example, Au or Cumay be used as non-magnetic spacer layers or the spacer layer may becomprised of a conducting oxide layer. (It should be understood that theterms underlayer and overlayer do not necessarily imply any particularorientation with respect to gravity.)

Performance of the MgO (or Mg—ZnO) tunnel barriers disclosed herein maybe improved through annealing, wherein performance refers to variousattributes of the tunnel barrier or associated device. For example,annealing a magnetic tunnel junction improves, in particular, itsmagneto-tunneling resistance; annealing a tunnel barrier improves, inparticular, its spin polarization. In particular by annealing thesetunnel barriers, tunneling magneto-resistance of more than 100% canreadily be achieved using methods of thin film deposition and substratematerials compatible with conventional manufacturing technologies.Annealing temperatures may be in the range from 200° C. to 400° C. oreven higher; however, the best tunnel barrier performance was obtainedfor annealing temperatures in the range from 300° C. to 400° C. The sameanneal that improves the tunneling magnetoresistance may also be used toset the direction of an optional exchange bias field provided by anantiferromagnetic exchange bias layer and may also be used to set adirection of a uniaxial magnetic anisotropy in the magnetic electrodes.

The preferred embodiments and implementations of the invention aredirected to MgO or Mg—ZnO tunnel barrier layers which are substantially(100) oriented or textured. Certain non-amorphous magnetic layers andtunnel barrier layers are polycrystalline and are comprised of grains orcrystallites which range in lateral extent from approximately onehundred to several hundred angstroms (e.g., 500 angstroms). Thus, theselayers and the overall film structure are what is commonly referred toas textured. Texturing implies that there is a predominantcrystallographic orientation of individual layers and/or the overallfilm structure, but that the grains are not perfectly aligned along oneparticular direction. Individual grains may not be precisely orientedwith their (100) direction along the normal to the film layer, but the(100) direction within individual grains may be oriented away from thenormal to the plane of the film by an angle that can vary from a smallfraction of one degree to several degrees or even tens of degrees forpoorly textured films. The angular range of these (100) directions canbe used to quantify the degree of (100) crystalline texture of the filmstructure and can be measured using various structural characterizationtechniques, including cross-section transmission electron microscopy andvarious x-ray diffraction techniques. There may also be present grainswhich are oriented in a completely different direction, but theproportion of these grains is small for the method of formation of themagnetic tunnel junction structures described herein. Note that thecrystalline grains are randomly oriented with respect to a directionwithin the plane of the substrate on which the film structures aregrown. It is the orientation or texturing of the film which is importantwith regard to the preferred embodiments herein. Whereas the maximum TMRis obtained for film structures which are highly textured, the TMR willbe increased to the extent that the film structure is textured. It ispreferred that the angular range of a (100) direction within the grainsbe within + or −20 degrees of the film normal, or more preferablywithin + or −10 degrees, and even more preferably within + or −5degrees. As used herein, the term “(100) oriented” should be understoodto include the aforementioned deviations from the ideal case, in whichall the grains are precisely oriented with their (100) direction alongthe normal to the film layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, which includes FIGS. 1A and 1B, is a schematic of a magnetictunnel junction formed in accordance with the prior art. FIG. 1A shows amagnetic tunnel junction with a reference and a storage ferromagneticlayer, and FIG. 1B shows a magnetic tunnel junction device with areference layer (formed from a synthetic antiferromagnet) and a storageferromagnetic layer.

FIG. 2A illustrates the sequence of layers that are deposited to form amagnetic tunnel junction having high tunneling magnetoresistance (TMR).

FIG. 2B is a cross sectional view of the magnetic tunnel junction thatis formed according to the methodology of FIG. 2A.

FIG. 2C is a schematic cross-section of the arrangement of the atoms inone of the layers of the textured magnetic tunnel junction formedaccording to the methodology of FIG. 2A.

FIG. 3 shows resistance versus field curves for major and minor magnetichysteresis loops for an MTJ with a MgO tunnel barrier which exhibitsmore than 340% TMR at room temperature (and more than 550% TMR at 2.6K).

FIGS. 4A and 4B are cross-sectional views of still other magnetic tunneljunctions of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The tunneling current in an MTJ is spin polarized, which means that theelectrical current passing from one of the ferromagnetic layers ispredominantly composed of electrons of one spin type (spin up or spindown, depending on the orientation of the magnetization of theferromagnetic layer). The tunneling spin polarization P of the currentcan be inferred from a variety of different measurements. Themeasurement most relevant to magnetic tunneling is to measure theconductance as a function of bias voltage for junctions formed from asandwich of the ferromagnetic material of interest and a superconductingcounter electrode (R. Meservey and P. M. Tedrow, Phys. Rep. 238, 173(1994)). These studies show that the spin polarization of the tunnelcurrent measured in this way can be simply related to the TMR close tozero bias voltage as first proposed by Julliere (M. Julliere, Phys.Lett. 54A, 225 (1975)). In such a model P is defined as(n_(↑)-n_(↓))/(n_(↑)+n_(↓)), where n_(↑) and n_(↓) are the density ofspin up and spin down states at the ferromagnet/insulator interface. Byassuming that the tunnel current is comprised of two independentmajority and minority spin currents and that these currents are relatedto the respective density of states of the majority and minoritycarriers in the opposing ferromagnetic electrodes, the TMR can beformulated by the relation TMR=(R_(AP)−R_(P))/R_(P)=2P₁P₂/(1−P₁P₂),where R_(AP) and R_(P) are the resistances of the MTJ for anti-paralleland parallel orientation of the ferromagnetic electrodes, respectively,and P₁ and P₂ are the spin polarization values of the two ferromagneticelectrodes.

Experimentally, it is clear that the magnitude of the TMR is extremelysensitive to the nature of the interface between the tunneling barrierand the ferromagnetic electrode. By changing the properties of theinterface layer, for example, by inserting very thin layers ofnon-magnetic metals between the ferromagnet and the insulator layers,the TMR can be dramatically altered. Based on such observations, mostexperimental data on magnetic tunneling have usually been interpreted byassuming that P is largely determined by the electronic structure of theferromagnetic interface layer essentially independent of the tunnelbarrier electronic structure. However, P can also be strongly influencedby the probability of tunneling of electrons, which depends not only ontheir spin but also on the tunneling matrix element. The tunnelingmatrix element is determined by the detailed electronic structure of theferromagnet, the tunnel barrier, and the interface between theferromagnetic electrode and the tunnel barrier. For the sameferromagnetic electrode, the polarization of the tunneling current Pvaries depending on the material and structure of the tunnel barrier.

The possibility of high tunneling magnetoresistance in MTJs formed fromFe/MgO/Fe sandwiches where the tunnel barrier is formed from crystalline(100) oriented MgO layers was theorized by W. H. Butler, X.-G. Zhang, T.C. Schulthess et al., Phys. Rev. B 63, 054416 (2001). High TMR couldresult from the very slow decay through the tunnel barrier of majorityelectrons of a particular symmetry for the (100) orientation of Fe/MgO.This also means that the polarization of the tunneling electrons shouldalso be very high. However, extensive experimental work by many groupsover a period of several years showed no evidence for improved tunnelingmagnetoresistance values using crystalline (100) MgO tunnel barriers ascompared to amorphous alumina tunnel barriers. It was speculated thatduring the formation of the MgO tunnel barrier, the surface of the lowerFe electrode became oxidized perhaps resulting in much lower TMR thantheorized.

In U.S. patent application Ser. No. 10/973,954 to Parkin titled “MgOtunnel barriers and method of formation” filed Oct. 25, 2004, which ishereby incorporated by reference, a method for forming MgO tunnelbarriers is described which gives rise to MTJs that exhibitextraordinarily high values of tunneling magnetoresistance. To preventthe oxidation of a lower electrode formed from Fe, a method of formingthe MgO barrier was developed in which a thin layer of metallic Mg wasfirst deposited on top of the Fe layer and then a layer of MgO wasdeposited on top of this Mg layer through the reactive sputtering of Mgin an Ar—O₂ plasma. Using this method of preparing the MgO barrier, veryhigh tunneling magnetoresistance values were obtained, much higher thanany previously reported values for any MTJ at room temperature. Forexample, FIG. 3 shows data for an MTJ with a MgO tunnel barrier whichexhibits more than 340% TMR at room temperature (and more than 550% TMRat 4K).

FIG. 2A illustrates this process of forming a MTJ 200, in which a tunnelbarrier 120 is formed by first depositing a thin Mg layer 122 followedby deposition by reactive sputtering of an MgO layer 124. As shown inFIG. 2B, it is more appropriate to view the MgO tunnel barrier as asingle layer 120′, since the layer 122 is oxidized to form MgO, with thelayers 122 and 124 becoming largely indistinguishable as a result. Forexample, the layers 122 and 124 are not distinguishable in across-sectioned slice of the device examined in a transmission electronmicroscope. The thickness of the resulting MgO layer 120′ is preferablyin the range of 3-50 angstroms, more preferably 3-30 angstroms, stillmore preferably 3-20 angstroms, and most preferably 4-15 angstroms.

FIG. 2A shows a device that includes a substrate 111, a bottomelectrical lead 112, an antiferromagnetic layer 116, a fixedferromagnetic (or ferrimagnetic) layer 115, a “free” ferromagnetic (orferrimagnetic) layer 134, and a top electrical lead 136, all of whichare similar to their FIG. 1B counterparts 11, 12, 16, 18, 34, and 36,respectively; these layers, as well as other layers and componentsreferred to herein, may be constructed using techniques known to thoseskilled in the art. The arrows 180 and 190 illustrate possibleorientations of the magnetic moments of the free ferromagnetic layer 134and the fixed ferromagnetic layer 115, respectively. As shown in FIGS.2A and 2B, the fixed ferromagnetic layer 115 may comprise more than onemagnetic layer and, for example, be comprised of a bilayer of twodifferent ferromagnetic layers 118 and 119, each having a magneticmoment oriented as indicated by the arrows 190 and 191, respectively.The layer 115 may also be comprised of a trilayer, 117, 118 and 119,each of these magnetic layers with a magnetic moment oriented along thedirection indicated by the arrows shown in the figure as 189, 190 and191. The magnetic layer 115, the antiferromagnetic layer 116, and thebottom lead 112 of FIGS. 2A and 2B constitute a lower electrode 110. Thelower electrode may be formed on an additional template layer 105.

MTJ structures formed according to the method described in U.S. patentapplication Ser. No. 10/973,954 to Parkin titled “MgO tunnel barriersand method of formation” (filed Oct. 25, 2004), exhibit very hightunneling magnetoresistance values of more than 160% at roomtemperature. However, the high tunneling magnetoresistance is derivednot only from using a method of forming the MgO tunnel barrier whichdoes not oxidize the lower ferromagnetic electrode, but also fromforming a crystalline structure in which the ferromagnetic electrodesdirectly above and below the (100) textured MgO tunnel barrier have abcc crystalline structure and are also textured in the (100)orientation. The layer 115 is preferably formed from a bcc alloy formedfrom one or more of Co and Fe. For example, layer 118 may be formed fromFe or CO₈₄Fe₁₆ and layer 119 may be formed from CO₇₀Fe₃₀. Thecrystallographic texture of these layers can be controlled by suitablechoice of the underlayers. For example layer 112 may be formed from abi-layer of TaN and Ta or from a layer of Ta alone. Layer 112 may alsocomprise other layers for improved growth of the MTJ device 200. Inparticular, layer 112 may be grown on a layer 105 of MgO which improvesthe growth and crystalline texture of the subsequent layers. Layer 116may be formed from an fcc antiferromagnetic alloy of Ir and Mn where thecomposition of Ir is less than ˜30 atomic percent. The IrMn layer growshighly oriented in the (100) orientation when deposited on the layer 112formed from Ta or TaN/Ta. The substrate 111 may be comprised of anamorphous material such as SiO₂.

Using this combination of underlayers, the layer 115, which may becomprised of one or more bcc Co—Fe alloys and nominally amorphous CoFeBlayers, is textured in the (100) orientation and the MTJ 200 displayshigh TMR. For example, an additional layer 117 may be formed from[Co_(1-x)Fe_(x)]_(1-y)B_(y) where the composition of the Co—Fe componentis chosen so that the corresponding alloy without B is typically bcc,and the B content is chosen so that this layer is normally amorphouswhen the layer is sufficiently thick and is not subjected to atemperature at which the amorphous alloy would crystallize and take up acrystalline structure. Thus the B content may range from 8 to 35 atomicpercent. The thickness of the layer 117 is chosen to be very thin, asthin as ˜3 Å. This layer serves two possible purposes. On the one handMn from the IrMn antiferromagnetic layer diffuses from this layerthrough the layer 115 to the tunnel barrier 120 when the MTJ device isthermally annealed. The layer 117 may serve to reduce the amount of Mndiffusion which is deleterious for the TMR exhibited by the device. Onthe other hand the layer 117 may serve to improve the smoothness,crystalline texture and/or the crystalline perfection of the MgO tunnelbarrier.

A method of forming Mg_(1-x)Zn_(x)O tunnel barriers is now described inconnection with FIGS. 2A and 2B; Mg_(1-x)Zn_(x)O tunnel barriers may beused instead of MgO tunnel barriers in the structures disclosed hereinto form alternative embodiments. (See also U.S. application Ser. No.10/982,075 to Parkin titled “Mg—Zn oxide tunnel barriers and method offormation” filed Nov. 5, 2004, which is hereby incorporated byreference.) The Mg_(1-x)Zn_(x)O tunnel barriers are formed by i) firstdepositing, for example, a Mg—Zn layer without any oxygen (so that thisMg—Zn layer covers the underlying ferromagnetic or ferrimagnetic layer),and then by ii) depositing, for example, a layer of Mg—Zn in thepresence of reactive oxygen during which process the previouslydeposited first Mg—Zn layer is oxidized, thereby forming the tunnelbarrier.

The Mg—Zn composition of the metal layer 122 does not need to be thesame as the Mg—Zn composition of the oxide layer 124. Indeed the layer122 can be formed from pure Mg and the layer 124 can be formed from pureZnO. Alternatively, the layer 122 can be formed from pure Mg and thelayer 124 from [Mg_(1-x)Zn_(x)]. Alternatively, the layer 122 can beformed from an alloy with a composition [Mg_(1-y)Zn_(y)], whereas thelayer 124 can be formed by the deposition of a layer of composition[Mg_(1-z)Zn_(z)] in the presence of reactive oxygen.

In general, to form a Mg—Zn oxide tunnel barrier according to preferredimplementations of the invention herein, it is only necessary that oneof the layers 122 and 124 include Mg and that the other of these layersinclude Zn.

The Zn concentration in the layer 122 can be higher or lower than thatof the layer 124. The concentration of Zn in the layer 122 is preferablychosen to optimize the growth of the Mg—ZnO tunneling barrier 120′ aswell as for the target resistance-area product (RA) value. More Zn willlead to an oxide barrier with a reduced tunnel barrier height and solower RA. Similarly, increasing the concentration of Zn in the oxidelayer 124 will also lead to lower tunneling barrier heights and so tolower RA values. For the optimal tunnel barrier with the highest thermalstability, it may be preferable to form the layer 122 from an alloy ofMg—Zn with less Zn or even from pure Mg. It may also be preferable toform a tunnel barrier by first depositing a layer of Mg or a Mg—Zn alloywith small amounts of Zn, then by secondly depositing a layer of[Mg_(1-x)Zn_(x)] in the presence of reactive oxygen (in which this layercontains a higher concentration of Zn), then by thirdly depositing alayer of Mg or [Mg_(1-x)Zn_(x)] with lower concentrations of Zn in thepresence of reactive oxygen. (In this case, Mg—Zn oxide tunnel barriersof two or more layers may be formed. These layers may be of the form[Zn_(1-x)Mg_(x)]O, in which the Mg atomic percentage is between 1 and100, or between 1 and 99.) In general it may be advantageous to form thetunnel barrier 120′ from a first layer of Zn or Mg or Mg—Zn, and then bydepositing a sequence of Zn or Mg or Mg—Zn additional layers, in whicheach of the additional layers is formed in the presence of reactiveoxygen. The amount of reactive oxygen may be varied from layer to layer.For example, it may be advantageous to have more oxygen for higherconcentrations of Zn. It may also be preferable to have less reactiveoxygen in the last additional layer onto which the ferromagneticelectrode 134 is subsequently deposited. The Mg—ZnO tunnel barrier 120′so formed may advantageously have a thickness of between 3 and 50angstroms.

High tunneling magnetoresistance values have been found for a widecomposition range of the ternary [Mg_(1-x)Zn_(x)]O oxides, although thevalues are not typically as high as those found for oxides without anyzinc. Typically, just as for MgO tunnel barriers, the TMR values wereincreased for thermal annealing at moderate temperatures, although thethermal stability was reduced compared to that of zinc-free MgO tunnelbarriers. The thermal stability is very sensitive to the oxidation stateof the [Mg_(1-x)Zn_(x)]O layer, so that the properties of the MTJs arestrongly dependent on the reactive sputtering conditions under whichthese oxide layers are formed, especially to the ratio of argon andoxygen in the sputter gas mixture.

The preferred embodiments and implementations of the invention aredirected to certain magnetic layers and MgO or Mg—ZnO tunnel barrierlayers which are substantially (100) oriented or textured. This is shownschematically in FIG. 2C, which shows the arrangement of atoms in a bccstructure oriented in the (100) direction with respect to the directionof tunneling of electrons. Layers 320, 321, 322 of atoms within anindividual grain are shown for rows of atoms oriented along the [010]direction in-plane (indicated by the arrow 315) and (100) directionperpendicular to the plane (indicated by the arrow 310).

The pending applications to Parkin referenced above describe MTJs withhigh tunneling magnetoresistance using MgO tunnel barriers which use bccCoFe or Fe electrodes; the use of amorphous ferromagnetic electrodes isdescribed in U.S. patent application Ser. No. 10/884,831 to Parkintitled “High performance magnetic tunnel barriers with amorphousmaterials” filed Jul. 2, 2004. As described above, however, it may alsobe useful to use ferromagnetic or ferrimagnetic materials which areneither bcc nor amorphous.

The read performance of MTJ devices for MRAM applications is stronglyinfluenced by the magnitude of the tunneling magnetoresistance. MTJswith amorphous alumina tunnel barriers are limited to tunnelingmagnetoresistance (TMR) values of up to ˜70% at room temperature and lowvoltage bias. The use of MgO tunnel barriers formed in accordance withthe methods described herein gives rise to TMR values as high as 220%(see Parkin et al., “Giant Tunneling Magnetoresistance at RoomTemperature with MgO (100) Tunnel Barriers,” Nature Mater. 3, 862-867(2004)) or as high as 340% as described herein and illustrated in theexemplary data of FIG. 3. However, the MgO (and Mg—ZnO) tunnel barriersherein are crystalline, and although the tunnel barriers can be highlycrystallographically textured in various orientations (for examples, 100and 111) depending on the use of different underlayers on which thelower ferromagnetic electrode is deposited, the MgO (or Mg—ZnO) layer iscomprised of individual crystallites or grains whose in-planeorientation varies from grain to grain. This means that the crystalstructure of the ferromagnetic electrode deposited on top of thisbarrier will, to a greater or lesser extent, be influenced by thestructure of the MgO (or Mg—ZnO) layer.

For ferromagnetic electrodes formed from bcc CoFe alloys (as describedin U.S. patent application Ser. No. 10/973,954 to Parkin titled “MgOtunnel barriers and method of formation” filed Oct. 25, 2004), thecrystal grains from which the CoFe layer is comprised grow in anepitaxial relationship with those of the underlying MgO (or Mg—ZnO)layer. Since CoFe alloys exhibit significant crystalline magneticanisotropy, this means that the magnetic moments of the CoFecrystallites will be oriented in different directions in the plane ofthe CoFe film. Moreover, the anisotropy can be significant. As the sizeof MTJ devices shrinks to deep sub-micron dimensions, the number ofcrystallites will be reduced so that there will likely be significantvariations in the magnetic switching characteristics (easy and hard axiscoercive fields and hard axis anisotropy) of the storage layer ofindividual MTJ elements. This effect can be mitigated by the use ofamorphous ferromagnetic layers, preferably formed from CoFeB alloys, asdescribed in the pending applications to Parkin titled “High performancemagnetic tunnel barriers with amorphous materials” (application Ser. No.10/884,831 filed Jul. 2, 2004) and “Magnetic tunnel junctions usingamorphous materials as reference and free layers” (application Ser. No.10/904,449 filed Nov. 10, 2004).

In certain preferred methods of the current invention, a highly orientedlayer of crystalline MgO (or Mg—ZnO) is first formed to provide aninterface with a ferromagnetic electrode (preferably formed from Co—Feor Co—Fe—B alloys) that displays very high tunneling spin polarization.This layer is crystalline and preferably highly oriented in the 100crystallographic direction. Moreover, this layer is in the form of adielectric oxide before a layer of an amorphous alkaline earth oxide(AEO) is deposited in a second step.

The giant tunneling magnetoresistance for MTJs with (100) orientedcrystalline MgO tunnel barriers is theorized to be due to coherenttunneling of the electrons across the tunnel barrier, so that one mightexpect crystalline ferromagnetic electrodes to be required whosecrystalline structure is matched and aligned with that of the MgO layer.In U.S. patent application Ser. No. 11/099,184 to Parkin entitled“Magnetic tunnel junctions including crystalline and amorphous tunnelbarrier materials” filed Apr. 4, 2005 it is shown that the requirementof coherently tunneling electrons across the entire tunnel barrier isnot needed and that rather the ferromagnet/MgO combination itself givesrise to highly spin polarized electrons.

In U.S. patent application Ser. No. 11/099,184 to Parkin it isdemonstrated that that the TMR of a MTJ with a bilayer tunnel barrierformed from crystalline MgO and amorphous Al₂O₃ is between the TMR of aMTJ having a single layer of MgO and the TMR of a MTJ having a singlelayer of Al₂O₃. The TMR is directly related to the spin polarization ofthe electrons tunneling from one FM electrode/dielectric interface tothe other dielectric/FM interface. Thus, the polarization of theCoFe/MgO interface is as much as 85% at low temperatures, whereas thatof the CoFe/Al₂O₃ interface is 55%. Using Julliere's formula (see Phys.Lett. 54A (3), 225-226 (1975)), the TMR may then be deduced using twodistinct polarization values for the ferromagnet/MgO interface and theAl₂O₃/ferromagnet interface. The polarization of the former is muchhigher than the latter.

The properties of an MTJ with a bilayer tunnel barrier of MgO/AEO may beunderstood by considering that the MTJ has two independent interfaceswith two corresponding spin polarization values. Using Julliere'sformula, the TMR may then be deduced using the polarization value forthe ferromagnet/MgO interface and that for the FM/AEO interface.

FIG. 4A shows an MTJ device of the current invention that has hightunneling magnetoresistance, with a tunnel barrier formed by firstdepositing a layer of MgO or Mg—ZnO 120′ (as described herein), followedby depositing a second dielectric layer of a alkaline-earth oxide 123.The layer 120′ may be formed by depositing a layer of Mg, which issubstantially free of oxygen, and then subsequently converting it to alayer of MgO when the AEO layer 123 is deposited in a subsequent step.(Likewise, a layer of Mg and Zn, which is substantially free of oxygen,may be converted to a layer of Mg—ZnO when the AEO layer 123 isdeposited.) Alternatively, the layer 120′ may be formed by the methodsdescribed in the preceding paragraphs, by first forming an ultra thinlayer of Mg and subsequently depositing a layer of Mg in the presence ofoxygen. Alternatively, a layer of MgO may be formed using a depositionmethod that does not substantially result in oxidation of the underlyingelectrode. The second dielectric layer 123 is also crystalline. For thestructure of FIG. 4A the MgO layer 120′ is highly textured with acrystal orientation oriented along (100). When the layer 123 is formedfrom SrO, the SrO layer is likely also highly textured and orientedalong (100), but because SrO has a much larger unit cell than MgO, theSrO cell may be under compressive stress and its unit cell may berotated with respect to that of the MgO unit cell. The detailedcrystallographic structure of the AEO layer, with respect to its unitcell size and orientation, will depend on the relative thicknesses ofthe MgO and AEO layers as well as the structure and composition of theadjacent magnetic electrodes.

Preferred structures and methods of forming certain MTJ devices are nowdescribed. The structures are formed by magnetron sputtering using anargon sputter gas at a pressure of 3 mTorr unless otherwise stated. Allthe layers are formed at ambient temperature. The MTJ device shown inFIG. 4A has an exchange biased reference electrode 110 formed beneaththe MgO (or Mg—ZnO)/AEO bilayer tunnel barrier (120′/123). The magneticstate of the reference electrode 110 remains unchanged during theoperation of the device. The antiferromagnetic layer 116 is used to setthe direction of the moment of the ferromagnetic layer 115 by exchangebias. The direction of the exchange bias field is set either during thefabrication of the MTJ device or by heating the device above theblocking temperature of the antiferromagnetic layer and cooling thedevice in the presence of a magnetic field that is sufficiently large toalign the moment of the layer 115 along a given direction. Although useof the antiferromagnetic layer 116 is preferred, the device may be builtwithout it. The direction of the reference electrode 115 is thenmaintained during the operation of the device by providing a uniaxialanisotropy field. This may be provided by the intrinsicmagneto-crystalline anisotropy of the layer 115, or it may be providedby the shape anisotropy of the reference electrode or by other means.

In FIG. 4A the direction 185 of the magnetization of the storage layer175, located above the upper layer of the tunnel barrier 123, ismaintained either parallel or antiparallel to that of the layer 115 (onthe other side of the bilayer tunnel barrier) during the operation ofthe device. The MTJ device of FIG. 4A may also be inverted, such thatthe reference ferromagnetic electrode is formed above the tunnel barrierand the storage layer is formed beneath the tunnel barrier.

Likewise, the device may be formed so that the AEO tunnel barrier 123 ispositioned above or below the crystalline tunnel barrier 120′. In somesituations, it may be preferable to first form an AEO layer followed bya crystalline layer of MgO (or Mg—ZnO).

As shown in US Patent applications to Parkin titled “High performancemagnetic tunnel barriers with amorphous materials” (application Ser. No.10/884,831 filed Jul. 2, 2004) and “Magnetic tunnel junctions usingamorphous materials as reference and free layers” (application Ser. No.10/904,449 filed Nov. 10, 2004), MgO layers can be grown crystalline andhighly 100 oriented when deposited on an amorphous underlayer.Similarly, MgO can be grown highly 100 oriented when deposited on anamorphous layer of alumina. Similarly, the AEO layer will also likelygrow crystalline and highly oriented when deposited on an amorphousunderlayer.

When the MgO layer is deposited on AEO, it may not be necessary to usean Mg underlayer, but the MgO layer may be directly deposited byreactive magnetron sputtering from a Mg target using an argon-oxygen gasmixture or by various other means including pulsed laser deposition,thermal or electron beam evaporation from an MgO target or by ion beamsputtering from an MgO target or from a Mg target in the presence ofreactive oxygen.

In FIG. 4A, the substrate 111 is formed from an amorphous layer of SiO₂formed on a silicon substrate. The underlayer or bottom electrical lead112 is comprised of 50 Å Ta, which is deposited on a layer 105 of 100 ÅMgO. The layer 112 may also comprise a layer of TaN deposited on top ofthe MgO layer. The TaN layer is formed by reactive sputtering of Ta inan Ar—N₂ mixture containing 6% N₂. An antiferromagnetic layer 116 of 250Å thick IrMn is deposited on the Ta layer by ion beam sputter depositionusing a beam of energetic krypton ions from an rf plasma ion source. Thesputtering target used to form the IrMn layer has a composition ofIr₂₂Mn₇₈. Next, a ferromagnetic layer 117 of 3 Å [CO₇₀Fe₃₀]₇₀B₃₀ isdeposited followed by a layer 119 of 60 Å CO₇₀Fe₃₀. The referenceelectrode 115 may comprise more than one CoFe or CoFeB layers of variouscompositions, e.g., there may be a layer 118 comprised of CO₈₆Fe₁₄. Themoments of the layers 117, 118 and 119 are parallel to one anotherbecause these layers are strongly ferromagnetically exchange coupled;thus they act as a single ferromagnetic layer 115. The directions of themagnetic moments of the layers 117, 118 and 119 are shown as the arrows189, 190 and 191, respectively in FIG. 4A. The reference ferromagneticlayer 115 may also be formed from a single ferromagnetic layer which maybe comprised of a Co—Fe alloy whose structure is bcc or from a layer ofpure Fe or from an amorphous CoFeB layer, for example, [CO₇₀Fe₃₀]₈₀B₂₀.

An MgO layer 120′ is then formed on top of the lower ferromagneticelectrode 110 using the method described in U.S. patent application Ser.No. 10/973,954 to Parkin titled “MgO tunnel barriers and method offormation” (filed Oct. 25, 2004), by first depositing a thin layer of Mghaving a thickness in the range of 3 to 20 Å, for example, followed in asecond step by the deposition of a layer of Mg in the presence ofreactive oxygen. The thickness of the second layer, which is comprisedof MgO, is typically in the range from 3 to 20 Å depending on thedesired resistance-area product, which can range up to more than10⁹Ω(μm)². For the device of FIG. 4A, a Mg layer 8 Å thick was used,followed by an MgO layer 25 Å thick formed by reactive magnetronsputtering using an argon-oxygen plasma containing 3 atomic percentoxygen. During the deposition of the MgO layer, the Mg underlayerbecomes oxidized so that the two layers form a single MgO tunnelbarrier. The exact composition of the MgO layer may differ slightly fromthe stoichiometric composition but Rutherford backscattering data oncompanion films of MgO, 500 Å thick, show that, within experimentalerror, the MgO layer contains 50 atomic percent 0 and 50 atomic percentMg. (Alternatively, the layer 120′ may comprise Mg—ZnO, as discussedabove.)

A layer of AEO 123 is then deposited on top of the crystalline layer120′ by reactive sputtering from an alkaline-earth metal target in thepresence of an argon-oxygen gas mixture. The proportion of oxygen in thegas mixture depends on the detailed configuration of the sputterdeposition system and the rate of flow of sputtering gas into thedeposition system. Typical concentrations of oxygen are in the rangefrom 2 to 10 atomic percent but preferred concentrations use as littleoxygen as possible. For the structure shown in FIG. 4A, the flow ofoxygen into the sputter chamber ranged from 1 to 7 percent of that ofthe flow of argon into the chamber. The AEO layer can also be formed bythe evaporation of the corresponding alkaline-earth metal from a Knudsencell in the presence of atomic oxygen formed from an rf discharge in anenclosed cavity within the deposition chamber. This method has theadvantage of using much lower concentrations of oxygen. In anothermethod, the AEO layer can be formed by ion beam sputter deposition froma metallic alkaline-earth target in the presence of reactive oxygen, forexample, formed within an atomic oxygen source. The AEO 123 may also beformed by sputter-deposition (either rf magnetron or ion beam) or bypulsed laser deposition from an insulating target of the alkaline-earthoxide.

The alkaline earth oxide can be formed from one or more of the alkalineearth elements Sr, Ca, and Ba. The alkaline earth oxides SrO, CaO, andBaO are insulators that have simple cubic structures of the rock-salttype similar to that of MgO but with larger crystallographic unit cellsizes. Most importantly, the band gap between the valence and conductionbands is smaller in energy than that of MgO. In particular, the band gapdecreases systematically with increasing atomic number from Mg to Ca toSr and to Ba. Thus the tunneling resistance also decreases as thealkaline earth oxide is varied from MgO to CaO to SrO to BaO forotherwise the same thickness of oxide, assuming that the Fermi energy ofthe magnetic electrodes is pinned mid-gap, so that the tunnel barrierheight is approximately half the band-gap. Since the tunnelingresistance increases exponentially with tunnel barrier height, evensmall decrements in tunnel barrier height can substantially decrease thetunnel resistance. The tunnel barrier resistance will also be stronglyinfluenced by the effective mass of the tunneling electrons. For MgO inthe (100) orientation with (100) oriented bcc Co_(1-x)Fe_(x) electrodes,the effective mass of the tunneling electrons is significantly reducedby perhaps an order of magnitude from the electron mass.

The alkaline-earth oxides form oxides which are thermally stable becauseof their strong affinity for oxygen (very high enthalpy of formation).The AEO may also be formed from oxides of combinations of one or morealkaline-earth elements.

The alkaline-earth layer 123 may also be formed by first depositing athin layer of the alkaline-earth metal and then plasma-oxidizing thislayer using reactive oxygen either from an atomic oxygen source or froman oxygen plasma created in the vicinity of the first deposited alkalineearth metal layer and preferably with the application of a bias voltageof a few volts to the sample substrate. When the AEO layer is formed ona magnetic underlayer, the alkaline-earth metal layer is preferablychosen to be of a thickness sufficient to substantially cover theunderlying magnetic layer so as to prevent the surface of theferromagnetic layer from being oxidized during the subsequent growth ofthe AEO layer.

The AEO layer 123 is preferably grown with a thickness in the range from2 to 50 angstroms and more preferably with a thickness in the range from2 to 20 angstroms.

Next, the MTJ device shown in FIG. 4A is completed by forming the topferromagnetic electrode 175, which is the storage layer. In FIG. 4A thestorage layer can actually be comprised of two ferromagnetic layers.First a thin layer of 20 Å CO₇₀Fe₃₀ is deposited on the MgO (or Mg—ZnO)tunnel barrier 120′. Second, an amorphous ferromagnetic layer is formed(over the thin CO₇₀Fe₃₀ layer), which is here comprised of 150 Å thick(Co₇₀Fe₃₀)₇₀B₃₀. This amorphous layer is formed by magnetron sputteringin a pure argon plasma where the target composition may vary slightlyfrom that of the deposited film. The preferred thickness of theamorphous CoFeB layer is in the range from 10 to 30 angstroms, and thepreferred thickness of the thin CoFe layer is in the range from 5 to 12angstroms. Alternatively, the storage layer 175 may be formed from asingle layer composed of either the thin CoFe layer or the amorphouslayer; in this case, the preferred thickness of the single layer is inthe range from 10 to 40 angstroms.

The storage layer may also be comprised of a synthetic antiferromagneticlayer comprised of two or more ferromagnetic or ferrimagnetic layerscoupled antiferromagnetically, in the absence of any applied magneticfield, by the use of a thin antiferromagnetic coupling layer such as athin layer of Ru or Os or a Ru—Os alloy or Cu.

Finally, the device 300 of FIG. 4A is completed by forming a cappinglayer 136 which is comprised of 100 Å Ta followed by 75 Å Ru. The layer136 may also be formed from TaN or from a combination of Ta and TaN. TheTaN layer is formed by reactive magnetron sputtering using anargon-nitrogen plasma containing about 8% nitrogen. The Ru layer isformed by ion beam sputtering. The amount of nitrogen in the sputter gasmixture used to form the TaN layer may be different for the growth ofthe capping layer 136 and the underlayer 112, in order to optimize thethermal stability of the structure and the crystallographic texture ofthe MgO (or Mg—ZnO) barrier for the highest tunneling magnetoresistancevalues. The capping layer may also be comprised of a thin oxide layer,such as a thin layer of MgO or alumina, which may improve the thermalstability of the device.

The TMR of MTJs having MgO barriers formed according to the currentinvention can be considerably increased by thermal annealing (see U.S.patent application Ser. No. 10/973,954 to Parkin titled “MgO tunnelbarriers and method of formation” filed Oct. 25, 2004). Similarly, theTMR of the MTJs of the current invention using MgO/AEO bilayers can beadvantageously increased by thermal annealing.

In another preferred embodiment of the current invention, the tunnelbarrier is formed from a trilayer 120′/123/125 comprised of MgO/AEO/MgO,as illustrated in FIG. 4B. After the deposition of the AEO layer 123, asecond MgO layer 125 is formed by methods similar to those used to formthe layer 120′ except that the Mg underlayer is not required. The layer125 may alternatively be comprised of Al₂O₃ which may be amorphous so asto form a trilayer barrier of the form MgO/AEO/Al₂O₃. The order of thelayers 123 and 125 may be reversed so as to form a trilayer barrier withthe structure MgO/Al₂O₃/AEO. In another embodiment the sequence of theselayers may be reversed so that the AEO layer or Al₂O₃ layer is formedfirst. It may be preferable to form the Al₂O₃ layer by first depositingan ultra thin layer of aluminum metal approximately 1.5 to 4 Å thickfollowed by deposition of an Al₂O₃ layer using, for example, reactivesputter deposition of aluminum in an argon-oxygen gas mixture, or bysome other method which provides oxygen (which may be molecular) tooxidize the first deposited aluminum layer without substantiallyoxidizing the underlying layer. The thickness of the deposited Al layeris sufficient to substantially cover the underlying electrode. Bycomparison with Mg, a thinner layer of Al is typically sufficient tosubstantially cover the underlying layer without resulting in oxidationof this layer during the subsequent oxidation of the metallic Al layer.(Note that Mg—ZnO may be used instead of MgO in one or more of thelayers of this device.)

While the preferred embodiments of the current invention apply tostructures with (100) texturing for the highest possible TMR or spinpolarization values, the structures and devices described herein may beprepared in other crystallographic orientations, such as (111), and sobe advantageous in other regards.

While the present invention has been particularly shown and describedwith reference to the preferred embodiments, it will be understood bythose skilled in the art that various changes in form and detail may bemade without departing from the spirit and scope of the invention.Accordingly, the disclosed invention is to be considered merely asillustrative and limited in scope only as specified in the appendedclaims.

1. A device, comprising: a tunnel barrier structure, the structureincluding i) a first layer that includes an alkaline earth oxide tunnelbarrier, wherein the alkaline earth oxide includes at least one of CaO,SrO, and BaO, and ii) a second layer that includes at least one of acrystalline MgO tunnel barrier and a crystalline Mg—ZnO tunnel barrier;and an underlayer in contact with the tunnel barrier structure, whereinthe underlayer, the first layer, and the second layer are in proximitywith each other, thereby enabling spin-polarized charge carriertransport between the underlayer and the first and second layers.
 2. Thedevice of claim 1, wherein the first layer is crystalline.
 3. The deviceof claim 1, wherein the underlayer includes at least one of Si and GaAs.4. The device of claim 1, wherein the underlayer includes a materialselected from the group consisting of ferrimagnetic materials andferromagnetic materials.
 5. The device of claim 4, further comprising anoverlayer that includes a material selected from the group consisting offerrimagnetic materials and ferromagnetic materials, wherein theunderlayer, the tunnel barrier structure, and the overlayer form amagnetic tunnel junction.
 6. The device of claim 4, wherein the secondlayer includes crystalline grains that are substantially (100) oriented.7. The device of claim 1, the tunnel barrier structure furthercomprising an Al₂O₃ tunnel barrier.
 8. The device of claim 1, whereinthe alkaline earth oxide includes an oxide of Ca.
 9. The device of claim1, wherein the alkaline earth oxide includes an oxide of Sr.
 10. Thedevice of claim 1, wherein the alkaline earth oxide includes an oxide ofBa.
 11. A device, comprising: a first magnetic layer; a second magneticlayer, wherein each of the first and second magnetic layers includes amaterial selected from the group consisting of ferrimagnetic materialsand ferromagnetic materials; a first tunnel barrier layer that includesan alkaline earth oxide tunnel barrier, wherein the alkaline earth oxideincludes at least one of CaO, SrO, and BaO; and a second tunnel barrierlayer that includes at least one of a crystalline MgO tunnel barrier anda crystalline Mg—ZnO tunnel barrier, the first and second tunnel barrierlayers forming a bilayer of tunnel barriers, wherein the first magneticlayer, the tunnel barrier bilayer, and the second magnetic layer form amagnetic tunnel junction.
 12. The device of claim 11, wherein the firstand second magnetic layers each include a Co—Fe alloy.
 13. The device ofclaim 11, wherein the device has a tunneling magnetoresistance ofgreater than 100% at room temperature.
 14. The device of claim 11,wherein the device has a tunneling magnetoresistance of greater than200% at room temperature.
 15. The device of claim 11, wherein the devicehas a tunneling magnetoresistance of greater than 300% at roomtemperature.
 16. A method of forming the device of claim 1, comprising:forming the second layer by: depositing at least one first metal onto asurface of the underlayer to form a metal layer thereon, wherein thesurface is substantially free of oxide; and directing at least onesecond metal, in the presence of oxygen, towards the metal layer to forma metal oxide tunnel barrier in contact with the underlayer, the oxygenreacting with the second metal and the metal layer, wherein the metaloxide tunnel barrier includes Mg; and forming the first layer by formingan alkaline earth oxide tunnel barrier over the metal oxide tunnelbarrier.
 17. The method of claim 16, wherein the metal oxide tunnelbarrier includes a MgO tunnel barrier.
 18. The method of claim 16,wherein the metal oxide tunnel barrier includes a Mg—ZnO tunnel barrier.19. The method of claim 16, further comprising annealing the metal oxidetunnel barrier to improve its performance.
 20. A method of forming thedevice of claim 1, comprising: depositing at least one first metal thatincludes Mg onto a surface of the underlayer to form a metal layerthereon, wherein the surface is substantially free of oxide; anddirecting at least one second metal that includes an alkaline earthelement, in the presence of oxygen, towards the metal layer to form abilayer in contact with the underlayer, the bilayer including the firstlayer and the second layer.